In the field of large cold storage freezers and similar devices, various systems such as solid doors, strip curtains, and air curtains, may be used to separate the cold storage room from an adjacent relatively warm anteroom. It is desirable to allow traffic from people and equipment through a doorway between the cold storage room and the adjacent warm room safely and with a minimum transfer of cool air from the cold room to the warm room.
The use of air curtains is one method of allowing a doorway to remain open to traffic while also preventing substantial energy loss between the cold and warm sides of the doorway. Air curtains generally direct air across the doorway to counter infiltration of warm air from the warm room to the cold room and exfiltration of cold air from the cold room to the warm room. By way of example, air curtains may direct air horizontally across the doorway or vertically, from an upper portion of the air curtain.
As a safety precaution, it is desirable to prevent the formation of fog, ice, and water in the doorway. Ice may form from the mixing of air from the cold and warm sides of the vestibule. The formation of ice at an air curtain depends on the temperature and relative humidity of the cold and warm rooms, and may be characterized by a plot on a psychrometric chart. The mixing of air from the relatively warm and cold sides may be characterized by a straight line between points representing the warm side temperature and humidity and the cold side temperature and humidity, which may be plotted on a psychrometric chart along with the saturation curve. Ice may form whenever the mixing line is to the left of, and above, the psychrometric saturation curve, as it is typically plotted.
The formation of ice may be prevented by heating the air discharged from the air curtain. By way of example, the discharged air may be heated to a temperature at a point on the psychrometric chart such that lines to such point from both the cold side and warm side temperature/humidity points remain to the right of, and below, the psychrometric saturation curve, as it is typically plotted.
While avoiding the formation of ice, water, and fog, it is also desirable to operate the air curtain as efficiently as possible, by adding the minimum amount of heat necessary to avoid such problems. With respect to the psychrometric chart, this means keeping the point representing the airstream with the added heat as close to the saturation curve as possible, without causing mixing lines from this point to the cold side and warm side temperature/humidity points to contact or cross the saturation curve.
Because temperature and humidity conditions in the cold and warm side rooms may change during operation of the air curtain arrangement, it is desirable in some applications to dynamically condition the discharged air in response to changing conditions. Conventional systems have various shortcomings. Some systems permit operation of the air curtain at points directly on the saturation curve. In changing environments, this permits the formation of ice, water, and fog because the system may not respond as quickly as the conditions change and because the sensors may not be sufficiently accurate for all positions in the doorway. This is particularly a problem for systems that rely upon mathematical approximations of the psychrometric saturation curve. Also, conventional control systems do not apply to multiple air curtain arrangements. A transitional vestibule may be formed between two or more air curtains positioned adjacent to each other across a doorway. Multiple air curtain arrangements—particularly those having two or more air curtains—present a more complex situation than do single air curtains. As a result, conventional systems for dynamically conditioning air do not operate efficiently, or even properly, in multiple air curtain arrangements.
Also, conventional air curtain arrangements provide limited ability to change the direction of airflow discharged across the doorway. Direction of the airflow across the doorway affects operation of the air curtain arrangement, and different operating environments may require different discharge angles in order to ensure efficient operation. Typically, airflow direction is fixed during installation and any adjustments made thereafter must be done manually. This is a time-consuming process that is difficult to perform properly by anyone other than a trained technician. Additionally, operating conditions for some air curtain arrangements change throughout the day, so it is impractical to have a trained technician manually adjust airflow operation.